I appear to be losing water out of the radiator over flow. Theres no evidence of oil in water or water in oil.
First question I have is on radiator caps. There appears to be next to no pressure when you press the cap down to tighten. And the spring is very easily compressed with your fingers. However I think these are only 7lb on the 850 so I guess there is very little resistance there?
The rad neck is 26mm deep so I cant try a standard higher pressure cap as theyre only 20mm.
Are there any other obvious symptoms to a blown head gasket?
First thing I would do is replace your cap like for like. I have had a cap ‘fail’ - I think the seal was just past it. Don’t try to second guess it until you have just tried a straight cap replacement. Also, how much are we talking about here? You will lose some if the coolant increases the pressure when hot to the point of opening the spring / seal. Does it continue to drop over time or is it a case of you noticing that the coolant has dropped a bit, topping up then noticing it has dropped again (repeat process) ? It may just be that you are filling to point that when hot it opens the cap a bit and then you are continually topping it up to that point. I would leave it a while to see if it continues to drop. In fact that is exactly what I am doing with my car just now as I seem to have the same situation as you!
Where can I get 15 lb caps in the 27mm length to suit an 850? Minis spares only seem to show the 7lb option. My later 1275 has a shorter radiator neck.
Also, as above, do not over fill your radiator to the brim. My '63 Super Delux had a piece of wire which went across the inlet about an inch below the inlet rim, which was the fill level. This is for expansion purposes.
+1 for replacing your cap first. Same thing was happing to me, but my cap even felt like it was strong enough so even the slightest wear to the spring will let out water.
Also, I’ve read that water is actually a better cooler than actual coolant.
